Reconstructed type tomography is used to present images of sections of a body. These images of sections are reconstructed from image projections obtained while a detector rotates about an axis passing through a body to be studied, perpendicular to the image sections to be produced. In the X-ray field, the image projection is generally a line image projection: the detector comprises a row of aligned detectors, facing the X-ray tube, in the plane of the section. In recent developments, however, the image projection is a 2D image projection: the detector has a plurality of detector cells arranged in two axes perpendicular to the main axis of X-ray emission.
In the field of nuclear medicine, the image projections are initially 2D images. These 2D image projections are related to the structure of the gamma cameras used in the tomography machines. A gamma camera is used in nuclear medicine for the display, in an organ, of the distribution of molecules marked by a radio-active isotope injected into a patient. Thus, a gamma camera has a collimator to focus the gamma photons emitted by the patient's body, a scintillator crystal to convert the gamma photons into light photons or scintillations, and an array of photomultiplier tubes, each of which converts the scintillations into electrical pulses. The scintillator and the collimator are normally constituted by flat plates: the image projections along a main orientation of the scintillator, perpendicular to its plane, are 2D images. A gamma camera further comprises electronic circuits for the production, from the electrical signals given by the tubes, of signals of coordinates X and Y of positions on the scintillator at which the scintillations are produced. A detection system such as this is followed by a processing and display unit that can be used to obtain an image projection of the distribution of the radioactive isotopes in the patient during the acquisition of the image.
Among other qualities, a gamma camera should possess high spatial resolution, namely the capacity to distinguish between small radioactive sources that are close to one another, and good response in terms of counting rate, namely the capacity to process a large number of gamma photons per unit of time. The spatial resolution depends on the accuracy of computation of the coordinates X and Y. The quality of the preparation of these coordinates depends essentially on physical laws that govern the working of the different parts of the gamma camera. Thus, the interaction of a gamma photon with the crystal gives rise to a light scintillation, the intensity of which decreases exponentially with time. The time constant of this decrease is characteristic of the scintillator crystal used. For a thallium-activated sodium iodide NaI(Tl) crystal, it is of the order of 250 nanoseconds.
A scintillation is seen by several photomultiplier tubes simultaneously. The light photons forming this scintillation liberate photoelectrons from the photocathodes of the photomultiplier tubes. For a scintillation with a given energy level, the number of photoelectrons thus liberated is governed by a Poisson statistical law. This means that the electrical signal delivered by a photomultiplier tube receiving a scintillation has an amplitude, the value of which follows a Posson statistical distribution and the mean value of which is a function of the energy of the incident light photons.
It can thus be assumed that a gamma ray gives rise, in a scintillator, to about ten thousand photons. The efficiency of the photomultiplier tubes is generally low, about ten per cent. The result thereof is that for a given scintillation, only several hundreds of light photons (for example 700) are detected by the photomultiplier tube. However, for these detected photons, the electrical signal delivered by the photomultiplier tube fluctuates according to the Poisson law referred to. This fluctuation is due to the mode of detection by the tubes: it relates to the liberation of electrons from the dynodes of the tubes. Owing to the quantum character of this liberation and the small number of effective instances of liberation, it is necessary to take account of the statistical phenomenon.
A scintillation is normally omnidirectional. The scintillator in itself absorbs a part of the energy from the scintillations before delivering the light photons in such a way that, as and when the distance from the place where the scintillation is produced increases, the light energy emitted by the scintillator decreases exponentially. This has two consequences: firstly, a scintillation will excite several photomultiplier tubes in this way. Secondly, this exponential decrease itself will be turned to advantage to enable the recomputation, from all the signals given by all the photomultipliers, of the place in which the scintillation is produced. A computation such as this is described, for example, in the French patent application No. 83 08825 filed on 27th May 1983. The image projections thus acquired commonly have a resolution of the order of 5 mm. This means that it is possible, with images such as these, to differentiate in the image between two objects at a distance of less than 5 mm from each other, or objects larger than 5 mm.
To carry out operations of tomography, it is necessary to acquire a number of these 2D image projections, while a main direction of examination of the gamma camera takes different orientations with respect to the body. These different orientations are obtained by mounting the gamma camera on a mount capable of making it rotate about an axis passing through the body perpendicularly to the sections to be produced.
The collimators used in the gamma cameras are regarded as a thick, flat plate made of a material that is opaque to the gamma rays in which elongated holes, all parallel to one another, are made. These holes are oriented perpendicularly to the planes of the collimator and of the scintillator. They make it possible to prevent the effects of scattering of the gamma rays, as well as to eliminate those gamma rays which would not propagate perpendicularly to the plane of the scintillator. Consequently, the scintillations occur normally in the scintillator in a position, in straight forward projection, vertical to the place where the gamma emissions have occurred in the body.
The implementation of the known algorithms for the reconstruction of tomographic images then makes it necessary to convert the 2D image projections thus acquired into line image projections. If it is assumed that the axis of rotation of the gamma camera is parallel to Y and if the 2D image projections possess pixels aligned parallel to Y, on the one hand, and parallel to X (perpendicular to Y) on the other hand, it is necessary, in order to acquire a line image, to take account only of those elements of the image projection which are located in an image band parallel to X. This band should always be chosen at the same place in the different image projections concerned. These line images enable the reconstruction of the image of an examined slice. This slice is, during the examination, vertically facing these bands.
According to a method such as this, it is possible theoretically to reconstruct as many sectional images in the body as there are bands likely to be made in the image projections. Thus, if an image projection is divided into 20 bands, and if, each time, the processing operations are made on corresponding bands, it is possible to obtain 20 sectional images. For an examined field of the order of 20 cm, images of slices are thus obtained with a thickness equal to 1 cm.
However, this is only the theoretical result. In practice, it can be shown that the real width of the slice taken into account by a band, with an electronic type of selection such as this, is widening from one side, the thickness of which is equal to the width of the band, up to the other side. The increase in the thickness of the slice varies with the width of the holes and varies inversely proportionally to the length of the holes. Furthermore, it can be shown that the sensitivity of detection varies enormously depending on whether the place from which the gamma emission emanates in the slice is close to the rotational axis of the gamma camera or is at a distance from it. The approach that would consist in reducing the dimensions of the holes comes up against the problem of an excessive drop in sensitivity: the solid angle that illuminates an elementary surface of the scintillator gets reduced, in effect, as the square of the reduction of the holes. Given the omnidirectional nature of the gamma rays emitted, the number of the gamma rays detected would be reduced correspondingly, which would diminish the efficiency of the gamma camera.